As the support post structure for guard fence, there is known a support post structure configured such that a ground plate is fixedly mounted to a lower part of a support post, and a plurality of anchors fix the ground plate. In the above described support post structure for guard fence, a linkage part of the support post and the ground plate may be easily broken due to stress concentration, and the anchor may be easily broken or withdrawn. Furthermore, since a plurality of anchors are required to fix the ground plate, it is time consuming to fix the support post. Furthermore, since the support post structure is not detachable and reinstallable, the guard fence has to stand all through the winter. As a result of this, the guard fence may be easily deformed and broken due to snow pressure.
There is another support post structure in which a lower end of a support post of a guard fence is connected to a ground plate via a hinge part, which allows tilting only in a limited direction. However, when the above described support post structure receives an impact toward a non-tiltable direction, the support post structure may be easily broken at the hinge part of the lower end of the support post. Furthermore, an anchor may be easily broken or withdrawn when an external force is applied in a direction other than a rotatable direction of the hinge part at the time of receiving the impact.
In order to deal with these problems, Patent Documents 1 to 3, which will be described below, disclose support post structures for guard fence that allow the guard fence to tilt in all directions when the guard fence receives a falling object caused by rockfall, avalanche, mudslide, or the like, and guard fences provided with the support post for the guard fences.
Japanese Unexamined patent application, Publication No. 2011-32829 (Patent Document 1) discloses a guard fence provided with a tiltable support post that can tilt in all directions. The tiltable support post is configured as follows. A support post anchor has been driven into the ground, a ground plate is grounded in such a manner that a top part of the support post anchor is passing through anchor hole of the ground plate. Two constituent elements of a universal joint are respectively fixed to the ground plate and a connecting sleeve wherein the ground plate and the connecting sleeve are coupled via the universal joint in which the two elements are joined. The support post anchor is threaded into a hole of the universal joint, and connected using a screw nut. The support post is fixedly inserted into the connecting sleeve, and the connecting sleeve abuts with a bolt that is provided to a restriction plate, thereby restricting the support post from tilting toward a mountain side. Using the tiltable support post, when the support post receives a falling rock or the like, the support post tilts toward a direction in which an external force caused the falling rock or the like is exerted, thereby relaxing a bending stress at the universal hinge joint, and avoiding the anchor from being broken or withdrawn.
Japanese Patent No. 3289238 (Patent Document 2) discloses a falling rock preventive method of pocket type. According to this method, tiltable support posts are provided that can tilt in all directions to support a wire mesh covering an entire grid formed by horizontal and vertical ropes. Japanese Patent No. 3619962 (Patent Document 3) discloses a falling rock guard support post that is a tiltable support post similar to that of Patent Document 2. The tiltable support post according to Patent Document 3 is configured such that a support post anchor has been driven into the ground, a rope linked to the support post anchor is inserted through a support post of a cylindrical shape to which a mounting plate including an anchor hole is fixed. The support post is erected onto the support post anchor, and a holding fitting (buffer member) is provided to an end of the rope exposed from an upper end of the support post, thereby fixing the support post to a slope. According to the tiltable support post according to Patent Document 3, when the support post receives a falling rock or the like and tilted, if a tension on the rope does not exceed a limit, the rope expands and relaxes impact energy and then contracts and restores the support post to the initial standing state. However, in this case, since the expansion and contraction of the rope is not large in amount, the restoring force to the initial standing state is small. If a pressure of the falling rock is large and the tension on the rope exceeds the limit, the holding fitting (buffer member) slides the rope from its retaining position to absorb the impact energy.